Because of the magnetoresistive effect, the resistance of a magnetoresistive element changes depending on the relative magnetization directions of a reference layer and a storage layer. For example, when the magnetization directions of the reference layer and storage layer are the same as each other (parallel state), the resistance of the magnetoresistive element is low. When the magnetization directions of the reference layer and storage layer are opposite to each other (anti-parallel state), the resistance of the magnetoresistive element is high. If the resistance in the parallel state is represented by RP, and the resistance in the anti-parallel state is represented by RAP, the value defined by (RAP−RP)/RP is called an MR ratio.
The MR ratio of a magnetoresistive element is preferably as large as possible to realize high reliability and high-speed performance. Therefore, the crystal structures of the reference layer, tunnel barrier layer and storage layer are formed as continuous. However, each of these layers is extremely thin; for example, each layer is a few nm thin. Thus, in fact, the theoretical MR ratio cannot be obtained. To compensate for this reduction in MR ratio, the enhancement of performance of a sense amplifier and the differential read technique which stores one bit in two magnetoresistive elements, etc., are adopted.
However, these techniques generate new problems such as increased chip size, power consumption and manufacturing cost.